Unitary full-range current-clearing fusible element



Oct. 7, 1969 R. E. KOCH 3,471,818

UNITARY FULL-RANGE CURRENT-CLEARING FUSIBLE ELEMENT Filed Dec. 13. 1967United States Patent 3,471,818 UNITARY FULL-RANGE -(lURRENT-CLEARINGFUSIBLE ELEMENT Robert E. Koch, Pittsfield, Mass, assignor to GeneralElectric Company, a corporation of New York Filed Dec. 13, 1967, Ser.No. 690,158 Int. Cl. Hlllh 85/04 U.S. Cl. 337-296 6 Claims ABSTRACT OFTHE DISCLOSURE A plurality of continuous electrically and physicallyparallel fusible filaments are respectively laterally separated andlaterally contiguous in longitudinally contiguous series sections. Inone form, a section of a ribbon of fusible material is slit or punchedout to separate the filaments which are integral in the remaining partof the ribbon. In another form, laterally separated fusible wires aretwisted together or braided to form the section of contiguous filaments.The separated filaments are preferably provided with low melting alloybeads.

This invention relates to electric fuses and more particularly toimproved full-range current clearing fusible elements for current fuses.

An electric fuse is a protective device for automatically interruptingexcess current flow. The principal part of a fuse is a fusible elementwhich melts when heated by excess current flow through it. Ina currentfuse, the fusible element is embedded in granular material such as sandso that an are or arcs formed when the element melts is rapidly cooledand the arc rapidly extinguished thus producing rapid currentinterruption or clearing. In fact, such fuses can interrupt current toofast and thus produce an objectionable momentary or transient inductivevoltage rise in the circuit.

This transient voltage is usually controlled byhaving the crosssectional area of the fusible element nonuniform so that it melts firstin the smallest cross section area and then burns back at a controlledrate as the cross sectional area increases. For example, the element maybe a tapered or stepped diameter rod or wire or a perforated or notchedribbon. While this is quite satisfactory for interrupting very highcurrents such as fault currents, it is diflicult to secure a fast enoughrate of fusible element burnback to reliably interrupt currents slightlyin excess of the rating, such as small overload currents.

Reliable overload clearing is usually accomplished by having two or moreparallel spaced fusible elements. In such an arrangement, one elementwill always melt first thus increasing the current in the remainingelements so that another one will melt neat thus producing a cumulativecascading action until all have melted. Rapid burnb ack then takes placeand results in interruption of the overload current. Often a low meltingalloy bead is placed around each wire to control the melting of thefirst wire on very small overloads. Upon melting, the bead material willalloy with the wire to form a lower melting point eutectic mixture.However, when subjected to fault currents such as may be produced byshort circuits the multiple parallel wire construction burns back toofast and thus can produce objectionable voltage rise.

An obvious way to reduce over-voltages yet obtain positive full currentrange clearing is to aggregate a conventional weakened ribbon fuse and aconventional multiple parallel wire fuse by connecting them in serieseither in separate casings or cartridges or in the same casing orcartridge. However, this is expensive.

In accordance with this invention there is provided a novel unitaryfusible element of continuous filaments See which has positive,controlled voltage full range currentclearing ability. The filaments maybe integral parts of a ribbon which in a certain longitudinal section orsections are later-ally separated by slitting or punching the ribbon orthey may be separated parallel wires which in a certain longitudinalsection or sections are consolidated into contiguous contact by twistingor braiding.

An object of the invention is to provide a new and improved fusibleelement for electric fuses.

Another object of the invention is to provide a new and improvedfullarange currentclearing fusible element for current limiting fuses.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawing and its scope will bepointed out in the appended claims.

In the drawing,

FIG. 1 is a plan view of -'a ribbon type fusible element embodying theinvention,

FIG. 2 is a modification of FIG. 1,

FIG. 3 is a similar view of a multiple wire type fusible elementembodying the invention, and

FIG. 4 another modified ribbon type fusible element embodying theinvention.

Referring now to the drawing and more particularly to FIG. 1, there isshown therein a length of ribbon conductor 1 composed of suitable goodelectrically conductive material such as silver. It is longitudinallydivided into a set of sections 2 which are similar to each otheralternated with sections 3 which are different from sections 2 but aresimilar to each other. The sections 2 have an overall generally circularshape such as would be formed by punching a circular hole in theribbon 1. However, they actually comprise la plurality of filaments 4,5, 6, 7, 8 and 9 formed by punching or cutting out intermediate portions10, =11, 12, 13 and 14. The central cutout 12 is the longest, the outercutouts 10 and 14 are the shortest and the intermediate cutouts 1 1 and13 are of intermediate length. The ends of the cutouts are shown squarebut this is not essential and they can be curved or rounded. Preferablysurrounding each of the six filaments 49, in one of the sections 2 whichis located in the center of the ribbon longitudinally, is a bead orbeads 15 of low melting alloy.

The operation of FIG. 1 is as follows: On the occurrence of a high faultcurrent, most if not all of the slotted sections 2 melt before themerged sections 3 melt and burnback proceeds from the slotted sectionsinto the merged or solid sections. This controls the voltage surge bycontrolling the rate at which resistance is introduced into the circuit.The solder bead 15 does not enter into the operation on high faultcurrent clearing.

When subjected to low current faults, melting takes place first in thearea of the solder bead 15. It is not important that one filament meltfirst and several can melt at the same instant. It is important for theindividual filaments to be small enough or that there be a high enoughnumber in parallel so that the last one to melt will have a high enoughcurrent flowing through it to effect rapid burnback. The individualfilaments in the slotted sections 2 do not immediately begin to burnbackupon melting but every current carrying filament separates slightly asit melts until the last parallel filament melts and then burnbackproceeds. Current will then transfer from one filament to the other orto several in parallel, and burn them back, until complete arcextinguishing takes place.

However, in the case of both 'high and low fault currents, burnback willproceed through several or many of the merged sections 3 before completecurrent interruption takes place.

The relative lengths of the filaments in sections 2 are not critical andin the modification shown in FIG. 2 the intermediate cutouts 11' and 13are made the same length as the central cutout 12. It is also notessential that the solder bead for initiating and localizing low faultcurrent melting be on the filaments and as shown in FIG. 2 a solder bead15 can be located at the center of one of the solid or merged sections3'. FIG. 2 is otherwise the same as FIG. 1 and separates essentially inthe same manner.

In the modification shown in FIG. 3, the fusible element comprises aplurality of separate wires 16, 17, 18, 19 and 20 which in a section orsections 21 are laterally spaced or separated, each one being providedwith a low melting alloy bead or beads 21'. In another section orsections 22 of the fusible element the individual wires are broughttogether by twisting or braiding them. In other words, the fusibleelement can 'have two sections as shown in FIG. 3 or there can be aplurality of sections 21 alternated with a plurality of sections 22.

The operation of FIG. 3 is somewhat different from the operation ofFIGS. 1 and 2 because the actual current carrying area is the same inboth sections 21 and 22. Therefore, high current will melt section 22first because the smaller surface area in contact with sand will provideless heat loss. However, the large effective diameter of section 22 willprovide a lower voltage surge per unit length than the small parallelspaced wires of section 21.

In the modification shown in FIG. 4, a fusible element 1' has aconventional high current clearing section 23 [formed by a series ofaxially spaced circular 1holes formed therein and it has a low currentclearing section 24 made up of subsections 25 in which filamentaryfusible elements are formed by cutouts. As a concrete example, theelement 1' may be .189" wide and .0033" thick. The holes in the section23 may be .125" in diameter. The subsections 25 may be .25" longseparated by .040" and the conducting filaments formed between cutoutsmay be .014" in width.

It will, of course, be understood that suitable lengths of the fusibleelements shown in FIGS. 1 through 4 may conventionally be spirally woundon a central support or core of any suitable gas evolving material, thewhole being embedded in a sand or the equivalent filled casing orcartridge with the ends of the fusible element connected respectively tothe external terminals of the fuse. It will likewise be understood thattwo or more such elements may be arranged in the above describedconventional manner in a single fuse, those elements being electricallyin parallel.

While there have been shown and described particular embodiments of theinvention, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention, andtherefore it is intended by the appended claims to cover all suchchanges and modifications as fall within the true spirit and scope ofthe invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A one-piece full-range current-clearing fusible element for a currentlimiting (fuse comprising an elongated flat fusible conductor having aplurality of perforated sections at substantially equally spaced-apartareas along its length and having at least one imperforate section ofsubstantially uniform cross section between each of said perforatedsections, each of said perforated sections comprising a plurality offiat filaments having their respective longitudinal axes generallyparallel to the longitudinal axis of said conductor, the respectiveopposite ends of each of said fiat filaments comprising an integral partof the respective imperforate sections disposed adjacent thereto wherebyjointless transition is afforded between said filaments and saidimperforate sections, the total cross sectional area of the filaments ineach of said perforated sections being less than the cross-sectionalarea of each of said imperforate sections, thereby to adapt said fusibleconductor to afford low overcurrent protection by causing it to fuse atsaid perforated sections when subjected to a sustained low overcurrentand to afford a controlled rate of burnback due to the heat-sink effectof said imperforate sections when caused to fail under high overcurrentconditions.

2. The invention defined in claim 1 wherein the flat filaments of eachperforated section are substantially iden tical in shape and number tothe filaments in each of the other perforated sections of the fusibleconductor.

3. The invention defined in claim 2 wherein the filaments of eachperforated section define a plurality of apertures in the fusibleconductor, the apertures in each of said perforated sections defining aplurality of overall generally circular holes in the conductor with thefilaments of said sections extending across each of said holes, wherebythe current carrying portions of some of the filaments in saidperforated sections are of different lengths so their respectiveburnback times under overcurrent oonditions are substantially differentand afford a uniform rate of burnback for the conductor as a whole whenit fails under a high overcurrent condition.

4. A fusible element as defined in claim 1 including a bead mounted onat least one of said filaments, said head being formed of an alloy thathas a lower melting point than the melting point of said filamentswithout said bead thereon.

5. A fusible element as defined in claim 1 including a bead mounted onat least one of said imperforate sections, said bead being formed of analloy that has a lower melting point than the melting point of saidimperforate section without said bead thereon.

6. A fusible element as in claim 1 in which said conductor has anotherportion of its length provided with longitudinally tapering cutout areasthat are of a different size than the removed portions of one of theperforated sections thereof, whereby the rate of burnback of said otherportion is made different than the rate of burnback of the portion ofsaid fusible conductor containing the perforated sections.

References Cited UNITED STATES PATENTS 569,803 10/1896 Scott 337-2961,239,876 9/1917 B urnham 337-293 X 1,631,669 6/1927 Brown 3372931,081,214 12/1913 Conant 337244 1,277,045 8/1918 Cole 337295 1,692,445ll/1928 Hope 337-293 X 3,138,682 6/1964 Dannenberg et al. 337295 XFOREIGN PATENTS 364,546 11/ 1962 Switzerland.

BERNARD A. GILHEANY, Primary Examiner H. B. GILSON, Assistant ExaminerUS. Cl. X.R.

